A brushless direct current (DC) motor is typically used to drive rotation of the spindle which carries a plurality of discs in a disc drive system. A brushless DC motor typically has a number of permanent magnets mounted on a rotor and a set of electromagnetic coils mounted on a stator. The rotor is made to rotate by energizing the coils in a specific sequence relative to the angular position of the rotor. The function of energizing the coils at specific points in the rotation is called "commutation" of the motor.
One prior method of motor commutation uses Hall sensors mounted on the stator for sensing the angular position of the rotor. Electronic circuits connected to the Hall sensors detect the passage of the rotor past the Hall sensors and switch the energy into a different electromagnetic coil (commutate) at that instant.
A second commutation method is based on back electromotive force (EMF) sensing. When the rotor rotates due to a coil being energized, an electromagnetic field is induced in the coils which are not currently energized. By sensing the resulting back EMF, the correct commutation state of the motor can be determined. Back EMF commutation has advantages in that it does not require the use of Hall sensors which tend to be large, and can be expensive.
The information derived from the back EMF is used to drive a voltage controlled oscillator (VCO) in a phase locked loop control circuit. The output of the VCO is used to precisely clock commutation control logic which correctly commutates the motor for maximum torque efficiency, and motor direction.
While this type of motor commutation control circuit provides an excellent means of precisely commutating the motor at the optimum time, it has a drawback in that it creates a very pure tone, jitter free, commutation signal. Also, this signal is used to feed the motor through highly efficient, fast switching, power field effect transistors. Thus, high rates of current change in the motor coils are induced, and can cause audible mechanical resonances at the commutation frequency, and at harmonics of the commutation frequency. Further, such a system can exhibit electromagnetic radiation at high enough levels that it can become a concern, especially in the high frequency harmonics of the exceptionally jitter free commutation signal provided. If the unwanted resonances and radiation reaches a sufficient amplitude, they can lead to undesirable audible noise, servo track following errors and read/write errors.
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.